Field of the Invention
The invention relates to an air cushion guide for sheet or web-formed material, in particular for printed paper sheets in a printing press, by which guided sheet or web-formed material is supported on a supporting air cushion above at least one guide body or guide member formed with nozzle openings through which air is blown between the guide body or member and the guided material; and a method of adjusting a flotation or suspension height of a sheet or web-formed material.
Such air cushion guides have been described, for example, in the published German Patent Documents DE 44 27 448 A1 and DE 42 42 730 A1. In a most varied form and design, they are used, among other purposes, for transporting freshly printed and yet wet sheets of paper in a delivery system of offset printing presses, for example, in a contact-free manner from a printing unit to a delivery pile or, in sheet turning or reversing devices, for transporting sheets to sheet transfer drums or the like, between two impression cylinders. In this regard, a problem arises that, depending upon the printing job, quite different types of paper, sometimes printed on both sides thereof, have to be fed safely, i.e., without smearing. However, with an air cushion guide of fixed characteristics, which are determined by the number and form of the nozzles and by the amount of air blown through the nozzles, this cannot be assured in a like manner for all types of paper.
In general, one would assume that the risk of smearing would be all the less, the greater the height at which the sheet floats above the guide members. This is not quite true, however, because, in air cushion guides which operate on the principle of the hydrodynamic paradox, stability of guidance depends upon the height of the air cushion. Thicker air cushions are less stable, i.e., the restoring forces exerted by the air cushion on the guided sheets when changes in spacing occur are much less thereat than in floating guides where there is only a slight spacing between the guide member and the sheets and where, because of a high flow speed of the air flowing out of the nozzles, the guided sheet is guided quite stably, that is, with high restoring forces. The latter is the more optimal solution especially for thin, yielding papers, whereas, a too small spacing from the guide baffles is problematic for stiff, thick paper qualiies.
It would therefore be optimal if one could realize an air cushion guide which simultaneously combines both a large spacing of the guided sheet and great stability because of a high flow speed of the blown or blast air under the sheet. It is self-deceiving, however, to assume that this could be achieved with an air cushion operating in accordance with the aerodynamic paradox, by simply "opening up the blower" and thus lifting the sheet by blowing a greater amount of air into the air cushion. This becomes clear from the graph in FIG. 4, wherein the operating conditions of an air cushion guide according to the prior art are shown. Specifically, the curve c thereof shows the dependency of the speed c of the airflow blown through the nozzles into the air cushion, and the curve Q shows the volumetric flow Q, respectively, dependent upon the initial or supply pressure P.sub.v of the chamber in the guide member from which the nozzles are supplied. If the pressure is increased, both variables vary approximately to the same extent or, in other words, approximately proportionally to one another. Conversely, the flotation height, as the curve h shows, remains virtually the same when pressure variations occur over a relatively wide range between 0.5 millibars (mbar) and 10 mbar.
Because it was consequently impossible to adjust the flotation height of the guided sheets to the various paper qualities by controlling the air compressor used for the air cushion guide, other courses were taken heretofore. For example, the hereinafore mentioned, published German Patent Document DE 42 42 730 A1 teaches disposing the air openings or nozzles in interchangeable replacement cassettes, i.e., matching the air cushion guide to the guide material is accomplished by replacing cassettes. This is unable to be effected during operation of the printing press, however, nor can it be automated.
In the published German Patent Document DE 42 09 167 A1, a sheet guiding device is described wherein the flotation height of the sheet in the middle portion thereof is increased by additional blower nozzles from which airstreams or flows are blown which strike the sheet surface perpendicularly and lift the sheet in the middle thereof by the impulse effect of these additional airstreams. Although this may possibly allow the flotation height or level to be set uniformly over the width of the sheet, it does not produce an overall change in the flotation height.
A combination of nozzles which operate in- accordance with the hydrodynamic paradox, and blower or blast nozzles directed perpendicularly to the guided paper web so as to increase the flotation height of the guided web and make it more uniform have been described for weblike materials hereinbefore in German Patent 17 74 126. However, this reference discloses no possible way of adapting or matching the flotation height to various material qualities during operation of the device.
From German Patent 20 20 430, it has become known heretofore for somewhat airfoil-shaped air cushion guide members, for guiding weblike materials, to be switched over mechanically in such a manner that at least two stable zones are produced for the spacing between the guided web and the guide member. The characteristic of the guide member is varied so that it acts, on the one hand, as an air cushion nozzle and, on the other hand, as an airfoil nozzle, i.e., in accordance with the hydrodynamic paradox. In this regard, however, the greater spacing of the guided part, which results from the air cushion characteristic, is achieved at the cost of reduced stability of the air cushion produced by this type of nozzle.